Experimental Study of Paraffin Deposition Under Two-Phase Gas/Oil Slug Flow in Horizontal Pipes

Abstract

Summary Paraffin or wax deposition under two-phase gas/oil slug flow in horizontal pipes is studied experimentally. The experiments are conducted by use of a 2.067-in., large-scale multiphase flow loop under an operating pressure of 350 psig. Testing fluids are Garden Banks condensate and natural gas. Two-phase-flow hydrodynamics and single-phase paraffin-deposition experiments are conducted and used as base cases for a comprehensive data analysis. Deposition experiments are conducted with parametric variation of oil and gas superficial velocities and testing durations of 4, 12, and 24 hours. The bulk fluid and initial pipe-wall temperatures at a removable spool piece are kept below wax-appearance temperature (WAT) and relatively constant to ensure consistent initial dissolved-wax-concentration differences and to avoid thermal restriction in the growth of the deposit. Unprecedented detailed measurement and analysis of the circumferential local samples are conducted by use of a newly designed pigging tool with high-temperature-gas-chromatography (HTGC) and differential-scanning-calorimetry (DSC) deposit-characterization techniques. The results reveal that deposit thickness decreases with the increase in the superficial liquid velocity (vSL) for all two-phase-flow cases. The crossover trends of the thickness vs. time are suggested by the single-phase deposition data to occur at times less than 6 hours. Circumferential analysis shows that, for the slug-flow wax deposit, the deposit at the top wall is thicker (by a factor of approximately 1.9), softer, and contains more oil than the bottom deposit. The final overall deposit wax contents are found to increase with the superficial gas velocity (vSG) for the case of vSL = 1 ft/sec, but nonmonotonic change occurs for the case of vSL = 3 ft/sec. Wax deposits under slug flow have longer chain n-alkane compared with single-phase flow having the same wax-fraction cases. Wax mass at the top wall is greater (by a factor of approximately 1.7) than the bottom mass. This new set of complete experimental data serves as the fundamental understanding of single-phase and two-phase gas/oil slug-flow paraffin deposition.